Viral infections induce multiple components of the host's immune response, one of which is the selection and expansion of virus-specific cytotoxic (CD8+) T cells. Members of the Picornavirus family of RNA viruses, however, induce only very low levels of CD8+ T cells, despite replicating to very high titers, suggesting the presence of one or more mechanisms by which these viruses could downregulate this response. For poliovirus, the prototypical picornavirus, the viral protein 3A mediates the inhibition of MHC Class I antigen presentation, and also prevents the secretion of antiviral cytokines. Our laboratory has found that the closely related enterovirus, coxsackievirus B3 (CVB3), is incapable of inducing a strong CDS* T cell response, and this proposal directly addresses whether the phenomenon occurring during a poliovirus infection could be responsible for this observation. SPECIFIC AIMS: (1) To determine if one or more CVB3 proteins can mediate a downregulation of MHC Class I antigen presentation and/or the general inhibition of the host cell secretory pathway, and carry out mutagenesis studies on these proteins to isolate mutants incapable of these functions. These studies involve the construction of dicistronic reporter constructs to analyze the localization of fluorescent marker proteins, and additional constructs to study MHC Class I antigen presentation in vitro in the presence and absence of various CVB3 proteins. (2) Utilize reverse genetics to construct and characterize potential vaccine strains of CVB3 harboring the mutations predicted to augment the host CD8+ T cell response. Viable viruses harboring the mutations found in Specific Aim 1 will be utilized in vaccination/challenge: studies in mice.